Devices called fiber optic rotary joints are well known. Fiber optic rotary joints allow optical signals to be transferred between fibers located, respectively, on members that are rotating with respect to each other, such as where one member is rotating and the other member is stationary. Devices of this type are categorized as an on-axis rotary joint when the communicating optical fibers are located along the axis of rotation. Such devices are categorized as an off-axis rotary joint if access, by the communicating optical fibers, to the axis of rotation or centerline is not possible or practical. The technology employed in these two types of rotary joins is quite different. The present invention concerns off-axis rotary joints.
Contactless fiber optic rotary off-axis joints have been developed as disclosed in U.S. Pat. No. 4,525,025, which is also assigned to the present assignee. The '025 patent discloses a fiber optic rotary joint which couples a pulsed optical signal across a rotary interface and includes an annular reflective wall formed on a stator and an optic fiber mounted on the stator having one end in close proximity, and tangential, to the annular reflective wall. A signal emitted by one of the optic fibers will be reflected along the annular reflective wall and is then received by the other of the optic fibers.
Actual joints constructed in a manner similar to that generally disclosed in the '025 patent have been limited to a rotor diameter of about 10-12 inches and data rates of about 50 megabits/sec. due to unacceptable propagation delays causing bit pulse-width distortion. There is a need for contactless fiber optic joints having rotor diameters of up to about 40-50 inches using pulsed optical signals having data transfer rates of up to about 100-400 megabits/sec. To meet these requirements, two criteria must be met. First, optical variations that occur due to the rotation of one of the members must be minimized. Second, propagation delays must be controlled to minimize their effect on pulse-width distortion.
Optical variations resulting from rotation can be minimized by using a multiplicity of optical pick-ups spaced circumferentially about either the rotor or the stator, respectively. The problem is that it is desirable to have as few pick-ups as possible in order to minimize complexity and therefore minimize cost. This has been addressed in U.S. Pat. No. 5,991,478 issued Nov. 23, 1999 and U.S. Pat. No. 6,104,849 issued Aug. 15, 2000. These patent are also assigned to the assignee of the present invention and patent application. The entirety of these two patents are hereby incorporated by reference into the present specification.
The present off-axis fiber optic rotary joint technology uses multiple inputs and pick-ups to send and receive data across members that have large diameters. The use of multiple inputs and pick-ups is required to keep the optical signals at a level that is sufficiently high to permit the photodiode receivers to operate. Wave guides are employed. The multiple inputs and pick-ups also cause a rapid rise and fall of the signal because the signal reflects from one area of the waveguide to another.
The inputs and pick-ups consist of lens/prism assemblies or other means of transmission that require very close mechanical tolerances when installed in the system. The inputs and pick-ups must pass very close to each other to maintain the proper optical levels. The costs associated with multiple inputs and pick-ups can make production prohibitive.